Net Loss Per Share - Basic and Diluted (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Numerator:
Net loss	$ (11,139)	$ (7,057)	$ (29,799)	$ (20,804)
Dividend attributable to down round feature of warrants				(303)
Dividend attributable to Series A & B convertible preferred stock				(372)
Series A & B convertible preferred stock - beneficial conversion feature				(9,399)
Net loss attributable to common stockholders for basic and diluted loss per share	$ (11,139)	$ (7,057)	$ (29,799)	$ (30,878)
Denominator:
Weighted average number of common shares outstanding - basic and diluted (in shares)	78,218,774	76,321,644	76,631,172	53,548,902
Net loss per share attributable to common stockholders - basic and diluted (in dollars per share)	$ (0.14)	$ (0.09)	$ (0.39)	$ (0.58)
Exercise of the pre-funded warrants			24,930,483